The invention relates to a method of operating a system comprising an internal combustion engine and an exhaust gas aftertreatment system, to a control unit for such a system, to an exhaust gas aftertreatment system, and to an internal combustion engine.
Exhaust gas aftertreatment systems comprising at least one catalyst for selective catalytic reduction of nitrogen oxides (SCR catalyst) are used to reduce nitrogen oxide emissions to legally stipulated values. Over a service life of the exhaust gas aftertreatment system, it is possible that the conversion rate that such an SCR catalyst can achieve will decrease. A typical design of a system comprising an internal combustion engine and such an exhaust gas aftertreatment system comprising an SCR catalyst takes account of this decrease in the performance of the SCR catalyst by means of a design reserve. This is also referred to as aging reserve. According to the specific application, the aging reserve may be very large in size. The effect of this is that the internal combustion engine in the new state of the system, with respect to the expected aging, is operated with raw nitrogen oxide emissions well below the emission level that could actually be run while complying with the legal limits by virtue of the as-new SCR catalyst, taking account of the aging reserve. These raw nitrogen oxide emissions, which tend to be too low, owing to the lack of exhaustion of the potential of the as-new SCR catalyst, lead to elevated consumption by the internal combustion engine, which is unfavorable for reasons of cost and also from environmental aspects. By means of recognition of aging, it is possible in principle to keep the nitrogen oxide emissions within the legally permissible range even in the case of aging of the SCR catalyst, but this does not solve the problem of excessive consumption with regard to the performance and conversion rate of a new SCR catalyst prior to commencement of the lifetime thereof.